Staircase Calculator

Design code-compliant stairs from total rise height. Calculates number of steps, riser height, tread depth, stringer length, and staircase angle — with UK and US building code checks.

The staircase calculator determines the correct number of steps, riser height, tread depth, total rise, and total going for a staircase between two floor levels — ensuring compliance with Building Regulations and achieving a comfortable, safe stair geometry. Stair geometry is one of the most regulated aspects of domestic construction because errors in riser height, tread depth, or headroom clearance lead to trips, falls, and building control rejection. In England and Wales, Approved Document K (Protection from Falling) mandates that for private domestic stairs: risers must not exceed 220mm; goings must be at least 220mm; the pitch must not exceed 42°; headroom must be at least 2.0m. The 2R+G rule (twice the riser plus the going should equal approximately 550–700mm) is the industry formula for comfortable stair geometry.

Winders, quarter-landings, and spiral staircases introduce additional geometric constraints — especially for winders, where Building Regulations require a minimum 50mm going at the narrow end and 270mm at the mid-point (measured 270mm from the narrow end).

Enter the total rise (floor-to-floor height in mm, measured from finished floor to finished floor, including floor finish thickness).
Enter the desired approximate riser height (typically 170–200mm for domestic stairs) and the available horizontal space (total going).
The calculator determines the optimal number of risers and goings, adjusted so all risers are equal height and all goings are equal depth.
It checks compliance with Approved Document K (UK) limits and flags any dimension that violates regulations.
Read the string (stringer) length, total going, handrail height (min 900mm above pitch line), and headroom clearance from your provided ceiling height.

Staircase geometry formulas

Number of risers = Round(Total Rise ÷ Desired Riser Height)

Actual riser height = Total Rise ÷ Number of Risers

Total going = Number of treads × Tread depth (number of treads = number of risers − 1, unless the bottom step is the same level as the lower floor)

2R + G = 550–700mm (comfort check: twice the riser plus the going; target 600–640mm for most comfortable stairs)

Stringer (string board) length = √(Total Rise² + Total Going²)

Pitch angle = arctan(Total Rise ÷ Total Going) — maximum 42° for domestic stairs (UK Approved Document K)

Example: 2,750mm floor-to-floor rise, target ~175mm riser. Number of risers = round(2750 ÷ 175) = 16. Actual riser = 2750 ÷ 16 = 171.9mm. For 2R+G = 620: G = 620 − 2×171.9 = 276mm going. Total going = 15 × 276 = 4,140mm. Pitch = arctan(2750 ÷ 4140) = 33.6°. All within UK limits. ✓

Reading your staircase calculation

UK Approved Document K compliance summary

UK Building Regulations (Approved Document K) limits for private domestic stairs: Maximum riser height = 220mm (min 150mm recommended). Minimum going = 220mm. Maximum pitch = 42°. Minimum headroom = 2.0m measured vertically from any pitch line point. Open risers (no riser board) permitted but nosing must overlap by min 16mm. Handrail height 900mm from pitch line; balustrade required where drop exceeds 600mm. For loft conversions (space-saver or alternating tread stairs): maximum pitch 42°, alternating tread stairs not permitted as sole means of escape. Winder treads: minimum 50mm at narrowest point; minimum 270mm measured 270mm from narrowest end. Spiral stairs: structural and access specialist design recommended.

Construction tips and best practices

Always measure floor-to-floor rise to the finished floor level — including floor finishes (tiles, carpet, screed) that will be added after framing. A 15mm tile finish can change the number of risers needed.
Use the 2R+G formula to balance riser and going: taller risers need shorter goings and vice versa. A 175mm riser pairs well with a 270mm going (2×175+270=620).
All risers must be equal height (±3mm tolerance) per Building Regulations — a single riser that is even 10mm different from the others causes a significant trip hazard and will fail Building Control inspection.
Headroom is measured at the tightest point — where the ceiling or floor above passes closest to the stair pitch line. Low ceilings at the top of the flight are a common non-compliance in loft conversions.
For timber staircases, always allow for glue, wedges, and settling when cutting strings — take your accurate measurements from the calculator to a qualified staircase joiner or use a pre-manufactured staircase kit.

The most comfortable domestic stair has a riser of approximately 175mm and going of 250mm (2R+G = 600mm) — a formula established by French architect François Blondel in 1675.
Falls on stairs cause approximately 700 deaths and 300,000 injuries per year in the UK — the most common cause of fatal accidents in the home.
A 2,500mm floor-to-floor rise using 175mm risers requires 14–15 risers, giving a total going of approximately 3.3–3.5m — the minimum hallway length needed for a straight flight.
Building Regulations have required open-riser balustrading not to have gaps exceeding 100mm (preventing a 100mm sphere passing through) since 1992 — this protects children from falling through or becoming trapped.

Common mistakes to avoid

Measuring floor-to-floor rise from unfinished floor levels and forgetting to add floor finish thickness — a 15mm tile finish on both floors changes the rise by 30mm, potentially requiring an extra riser.
Setting all treads equal but forgetting the bottom step — the first riser at the bottom is part of the total rise calculation; omitting it results in all risers being slightly too high.
Calculating headroom from floor to ceiling rather than from the pitch line — headroom must be measured perpendicular to the pitch line at every point along the flight, not just at the top landing.
Using the number of treads instead of risers for the total going calculation — a stair with 14 risers has 13 treads (if the top riser lands on the upper floor level) or 14 treads if there is a bottom step separate from the lower floor.

Staircase design in the UK is subject to Building Regulations Approved Document K (Protection from Falling, Collision and Impact). Any new staircase in a domestic dwelling, including loft conversions and extensions, requires Building Regulations approval from your Local Authority or an Approved Inspector. Loft conversion stairs with restricted headroom may qualify for relaxed Approved Document K standards under the "space-saving staircase" provisions — confirm applicability with your Building Control Body. This calculator provides geometric guidance only; final stair design must be reviewed and approved by a qualified Building Control surveyor or structural engineer.

Frequently Asked Questions

What is the ideal riser height for stairs?

The ideal riser height is 175mm (7 inches) for residential stairs. UK Building Regulations allow 150–220mm; US codes typically allow up to 7¾ inches (197mm).

What is the 2R + T rule for stairs?

The comfort rule states: 2 × Riser + Tread = 600–650mm (or 24–26 inches). A good stair design balances riser height and tread depth so each step feels natural to climb.

How do I calculate the number of stairs?

Divide the total floor-to-floor height (rise) by your desired riser height, then round to the nearest whole number. Recalculate the actual riser: actual riser = total rise ÷ number of steps.

What is the minimum tread depth for stairs?

UK regulations require a minimum going (tread depth) of 220mm for private stairs. US codes typically require a minimum of 10 inches (254mm).

How do I calculate stringer length?

The stringer is the diagonal board supporting the stairs. Length = √(total rise² + total run²), where total run = number of treads × tread depth.